A P25 tower won't broadcast a TG unless a radio on that specific TG is affiliated (connected) to that tower.
Back to your original question, if you want to hear Nampa FD, you'll have to listen to the tower that the fire dept radios will be on, which is likely the Nampa tower.
To hear the most Canyon county radio traffic, you'll have to listen to a radio tower in Canyon county. To hear the most Ada county radio traffic, you'll have to listen to a radio tower in Ada county.
P25 radios are constantly checking the signal strength of all the towers they can receive. Once a radio find a tower with a better signal strength, the radio will switch to that tower automatically. That's why sometimes you'll hear neighboring agencies on a tower in another county. Once that radio travels further away and connects to another tower, the tower in the other county will stop broadcasting that TG.
So, for example, if a guy from Nampa FD takes his family out to dinner in Ada county, and has his radio turned on, on a Nampa FD TG, once he gets close enough to a tower in Ada county and his radio affiliates with that tower, the tower in Ada county will start broadcasting the Nampa FD TG. When he turns off his radio, or when he goes back home to Canyon county, and his radio affiliates with a Canyon county tower, the Ada county tower will stop broadcasting the Nampa FD TG.